1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a digital multi-functional system including a combination thereof, and more particularly, to a sheet transport device that transports a recording medium such as a recording sheet.
2. Description of the Background Art
In general, image forming apparatuses, such as printers, facsimile machines, and copiers have a similar if not the same configuration as that illustrated in FIG. 1. FIG. 1 is a cross-sectional view illustrating a generally known image forming apparatus and a sheet transport path for a recording medium that includes but is not limited to a paper sheet (hereinafter simply referred to as a recording sheet).
When an image is formed on one side of a recording sheet P, the recording sheet P stacked in a sheet feed unit 3 is fed to an image forming unit 2. In the image forming unit 2, the image is transferred onto one side of the recording sheet P. Subsequently, the image is fixed by a fixing device 4, and then discharged to a sheet reversing portion, for example, a sheet reversing device 6.
In the sheet reversing device 6 serving as the sheet reversing portion, the recording sheet P is inverted. Then, the recording sheet P is discharged through a sheet discharge unit 8.
In this type of image forming apparatus, when an image is formed on both sides of the recording sheet P, the recording sheet P stacked in the sheet feed unit 3 is fed to the image forming unit 2 in which the image is transferred on one side of the recording sheet P.
Subsequently, the image is fixed on the recording sheet P in the fixing unit 4 and discharged to the sheet reversing device 6 in which the recording sheet P is inverted. The recording sheet P is then supplied again to the image forming unit 2 through a duplex sheet feed path 7. In the image forming unit 2, the image is formed on the other side of the recording sheet P.
When, on the other hand, an image is formed on only one side of the recording sheet P, the recording sheet P stacked in the sheet feed unit 3 is supplied to the image forming unit 2, and the image is transferred onto one side of the recording sheet 2. Subsequently, the image is fixed on the recording sheet P in the fixing unit 4, and then discharged through the sheet discharge unit 8.
It is to be noted that, in FIG. 1, reference numeral 5 refers to a sheet delivery unit, 41 refers to a pair of sheet transport rollers, 16 refers to a discharge-direction switch unit, 15 refers to a turn roller, and 30 refers to a sheet transport direction switch unit.
Referring now to FIG. 2, there is provided a schematic diagram illustrating the sheet reversing device 6 including reverse-transport rollers and peripheral components thereof in the image forming apparatus of FIG. 1.
After the image is transferred and fixed on the recording sheet P, the recording sheet P is transported to the sheet reversing device 6.
The sheet reversing device 6 includes reverse-transport rollers 90, sheet transport claws 100, and a reverse table 11. In the sheet reversing device 6, the reverse-transport rollers 90 and the sheet transport claws 100 draw out and send the recording sheet P to the reverse table 11.
With reference to FIG. 3, a description will be provided of an example of transport of the recording sheet P. In FIG. 3, each of the sheet transport claws 100 having a comb-like shape is alternately aligned with each of the reverse-transport rollers 90 on a shaft 190.
As illustrated in FIG. 3, the reverse-transport rollers 90 and the sheet transport claws 100 ruffles the recording sheet P, generating resilience and thereby pressing the recording sheet P against the reverse-transport rollers 90. Accordingly, the recording sheet P is transported reliably to the reverse table 11.
Referring now to FIG. 4, there is provided a side view illustrating the related art sheet transport claws 100 and the related art reverse-transport roller 90. As illustrated in FIG. 4, when the sheet transport claws 100 and the reverse-transport rollers 90 transport the recording sheet P, a spring 23 adjusts pressure against the recording sheet P.
In FIG. 4, a guide member 22a guides the recording sheet P as the recording sheet P advances into the reverse-transport rollers 90.
A pair of jogger fences 12 is provided on the reverse table 11. The pair of jogger fences 12 aligns and adjusts a position of the recording sheet P, and is movable in a vertical direction on the plane of FIG. 2.
When the recording sheet P is transported onto the reverse table 11, the pair of jogger fences 12 waiting at a position outside a sheet width travels to a position of guiding the recording sheet P from both sides thereof. While the pair of jogger fences 12 guides the recording sheet P, an invert driven roller 13 that has been moved substantially above descends to an invert drive roller 14 to contact and invert the recording sheet P. The recording sheet P is then transported to the turn roller 15.
It is to be noted that, in FIG. 2, reference number 34 refers to a separation claw, 35 refers to an intermediate inversion roller, and 36 and 37 refer to driven rollers.
Subsequently, the invert driven roller 13 rises so as to separate from the invert drive roller 14, thereby releasing the recording sheet P. When an image is formed on one side of the recording sheet P, the recording sheet P is transported to the sheet discharge unit 8 and discharged outside the image forming apparatus 1.
By contrast, when an image is formed on both sides of the recording sheet P, the turn roller 15 turns the recording sheet P and transports the recording sheet P to the duplex sheet feed path 7. Subsequently, the duplex sheet feed path 7 transports the recording sheet P to relay rollers 26, 27, and 28, thereby feeding the recording sheet P to the image forming unit 2 again.
FIG. 5 is a perspective view illustrating the related art sheet transport device 500 including the reverse-transport rollers 90 and the sheet transport claws 100. FIG. 6 is a cross-sectional view illustrating the related art reverse-transport roller 90 and the related art sheet transport claws 100 of FIG. 5 when the recording sheet P is transported to the reverse-transport roller 90. FIG. 7 is a cross-sectional view illustrating the reverse-transport roller 90 and the sheet transport claws 100 when the recording sheet P is being inverted.
As illustrated in FIG. 5, the reverse-transport rollers 90 rotate in one direction indicated by arrow C for the following reasons. For example, time loss caused by a recording sheet P being transported to the reverse table 11 and a recording sheet P2 being transported out of the reverse table 11 passing each other as illustrated in FIG. 7 can be reduced.
Further, in order to simplify a drive mechanism, the reverse-transport rollers 90 are configured to rotate in one direction so that the reverse-transport rollers 90 can be operated by a common motor that operates also other rollers.
Further, in order to create a ruffle shape on the recording sheet P as illustrated in FIG. 3, the sheet transport claws 100 are urged upward in a manner as illustrated in FIG. 4. The tip of each of the sheet transport claws 100 is disposed such that the tip thereof is positioned more inward than the peripheral surface of the reverse-transport rollers 90. In other words, a portion 90a illustrated in FIG. 7 of the reverse-transport rollers 90 projects substantially below the sheet transport claws 100.
As illustrated in FIG. 6, when the recording sheet P1 is transported onto the sheet reverse table 11, the recording sheet P1 presses down the sheet transport claws 100 so that the portion 90a is not generated or a relatively small amount of the portion 90a is generated. When the portion 90a is generated slightly, transport of the recording sheet P is not adversely affected.
However, when the recording sheet P is inverted, a corner of the recording sheet P is curled as illustrated in FIG. 5 and the curled tip portion P1 of the recording sheet P contacts the reverse-transport roller 90 rotating in an opposite direction as a transport direction indicated by arrow X.
In particular, the curled tip portion P1 of the recording sheet P contacts the portion 90a projecting below the sheet transport claw 100, causing the curled tip portion P1 to curl even more and resulting in paper jams and damage to the recording sheet P, for example, undesirable folding at the corner of the recording sheet P.
Further, when the image forming apparatus 1 fixes the image using heat, deformation of the recording sheet P such as curling tends to be aggravated after fixing process. In addition, such deformation of the recording sheet P occurs as the image is developed with toner.
According to the related art sheet transport device, such a reverse-transport roller is driven by a dedicated drive source, or is driven by a drive source of the sheet feed unit that is different from a drive source for a sheet transport device for reading operation, thereby increasing a number of drive sources employed in the image forming apparatus, and thus increasing the cost of the image forming apparatus as a whole.